Let me preface my comments with the statement that I too am undiagnosed (with ADHD, but have other mental health stuff going on) and am also trying to get an assessment. I also mean this post kindly :) I'm not sure that a positive response to dex necessarily indicates ADHD because most people will experience mild cognitive (and other) benefits from a low dose of amphetamines- that's one reason they're so addictive. Indeed chasing the feeling you describe can led to all sorts of trouble.. trust me. That of course doesn't you mean you don't have it!
I don't have any advice re: what to say because I haven't done it. But I'm certain that anything perceived as even remotely drug seeking is a big no-no. When I can eventually be seen I intend to to try and identify the things in my life that I believe are solely due to ADHD and not my other conditions.